1. Field of the Invention
The present invention relates to an image reading apparatus, multifunction printer apparatus, and image processing method. Particularly, the present invention relates to an image reading apparatus, multifunction printer apparatus, and image processing method which correct density or brightness represented by image data obtained by optically reading an image original.
2. Description of the Related Art
A color scanner is known as an image reading apparatus which reads an image by switching light of different light emission wavelengths. Such a color scanner has a linear light source and an image sensor provided on a carriage movable in a predetermined direction. The light source includes LEDs capable of irradiating light emission wavelengths corresponding to three primary colors of light, that is, red (R), green (G), and blue (B). The carriage is moved in a direction (sub-scanning direction) perpendicular to the elongated direction (main scanning direction) of the linear light source. The liner image sensor receives reflected light obtained by irradiating an image original with light and reads the image original. To read the image original, a scanning read method is employed.
In the scanning read method, an original is read by switching three LEDs serving as a light source while conveying a CIS (Contact Image Sensor) unit in the sub-scanning direction. More specifically, the R component data of a color image is obtained by lighting a red LED. Next, the G component data is obtained by lighting a green LED. Finally, the B component data is obtained by lighting a blue LED. Image data of one line is obtained in one red, green, and blue LED lighting cycle. Image data of one page of the image original is obtained by repeating the lighting cycle while conveying the CIS unit in the sub-scanning direction.
In scanning read in which the red, green, and blue LEDs are sequentially turned on, color misalignment occurs. As a method of reducing color misalignment, a method of performing reading by turning on two LED light sources between charge readout timings is known, as disclosed in Japanese Patent Laid-Open No. 2005-184390.
If bright LEDs are used to improve the signal-to-noise ratio, the cost increases. To solve this problem, a method of reading an image by simultaneously turning on two LED light sources is known, as disclosed in Japanese Patent Laid-Open No. 2006-197531.
Alternatively, an image forming apparatus described in Japanese Patent No. 3750429 is known, which performs a reading operation appropriate for an original by switching illumination light in accordance with the original type such as a negative original or positive original.
In these image original reading methods, the actual reading image characteristic includes variations in the manufacture of the image reading apparatus. LEDs used as a light emission source include variations in the forward voltage, variations in the current-light emission characteristic, and temperature dependency. The characteristic also changes depending on the emission color.
For these reasons, the quality of a read image can change variously depending on the apparatus that has read it.
To reduce the variations between apparatuses, for example, Japanese Patent Laid-Open No. 2005-184390 proposes a method of calibrating the LED light emission characteristic of each apparatus by setting, as the ON pulse width of each LED, a pulse width that makes the output amplitude match a predetermined value.
FIG. 16 is a time chart showing a primary color reading method of reading an image original by lighting only one color LED at a single timing.
As shown in FIG. 16, according to this method, the red (R), green (G), and blue (B) LEDs are sequentially turned on so that the respective color component data are output in synchronism with a pulse signal SH. When the red LED changes from ON to OFF, and the pulse signal SH is turned on, R component data is output. Similarly, when the green LED or blue LED changes from ON to OFF, and the pulse signal SH is turned on, G component data or B component data is output.
Let (R,G,B)=(255,255,255) be the brightness value of a read white original, and (R,G,B)=(0,255,255) be the brightness value of a read cyan original.
When an edge at which an original changes from white to cyan is read at the timing shown in FIG. 16, the output data of a line (a) is (R,G,B)=(255,255,255), and the output data of a line (b) is (R,G,B)=(0,255,255). In the line (a), at the light emission timing of the red LED, the original color is white. Hence, the brightness output value of the R component is 255. At the light emission timings of the green and blue LEDs, the original color is cyan. Hence, the brightness output value of the G component is 255. The brightness output value of the B component is also 255.
FIG. 17 is a time chart showing a complementary color reading method of reading an image original by simultaneously turning on two color LEDs (simultaneously lighting two primary colors).
When an edge at which an original changes from white to cyan is read by the complementary color reading method at the timing shown in FIG. 16, the brightness output values are as follows. The output data of a line (c) is (RG,GB,BR)=(510,510,255), and the output data of a line (d) is (RG,GB,BR)=(255,510,255). The read data is converted into the brightness values of the R, G, and B color components by equation (1). In the line (c), (R,G,B)=(128,255,128). In the line (d), (R,G,B)=(0,255,255).
                              (                                                    R                                                                    G                                                                    B                                              )                =                              1            2                    ⁢                      (                                                                                -                    1                                                                    1                                                  1                                                                              1                                                                      -                    1                                                                    1                                                                              1                                                  1                                                                      -                    1                                                                        )                    ⁢                      (                                                            GB                                                                              BR                                                                              RG                                                      )                                              (        1        )            
Using the obtained values of the lines (a) to (d), a CTF (Contrast Transfer Function) is calculated by equation (2). In the primary color reading method, CTF=18%. In the complementary color reading method, CTF=7%.
                              C          ⁢                                          ⁢          T          ⁢                                          ⁢          F                =                                                            W                p                            -                              B                p                                                                    W                p                            +                              B                p                                              ·          100                                    (        2        )            
Note that, in equation (2), Wp is the maximum brightness, and Bp is the minimum brightness.
As understood from a comparison between the calculated CTFs, the CTF value obtained by the complementary color reading method is smaller than that obtained by the primary color reading method. That is, if the complimentary reading method is employed, an image having a blurred edge is read.
The receiving side of the reflected light of each LED also suffers the influence of the transfer characteristic of the optical system and the characteristic of the photoelectric transducer. The variations on the side of the LEDs serving as a light emission source and those on the light receiving side combine together to yield many combinations of variations in individual image reading apparatuses and many interactions therebetween.
As described above, the conventional image original reading method does not consider the variations on the light receiving side of the image reading apparatus, the combinations of the variations on the light emitting side and those on the light receiving side, and the interactions therebetween. It is therefore impossible to sufficiently absorb the variations in the image characteristic of an entire image reading apparatus.